In the automotive industry, increasingly complex hollow sections are used to meet the requirements which are imposed in respect of strength and weight in so-called space frame structures. In addition to the forming of straight bead welded pipes into closed hollow sections which are designed to be load-compatible, diverse methods are known from the prior art for the production of hollow sections. A “rolling” method known from German Offenlegungsschrift DE 198 27 798 A1 belonging to the Applicant allows a particularly flexible, yet economic production of load-compatible hollow sections. In this method, a blank is formed in a slotted chamber whereby the blank is introduced by its longitudinal edges respectively into semi-cylindrical form gaps of two halves of a form tool which are in a mutually laterally reversed position and the longitudinal edges emerging from the form gaps in the apex of the form tool are welded together. During welding, the longitudinal edges are held in position by the halves of the form tool. One disadvantage of this method is that a tool which has form gaps, i.e. a tool with an internal mandrel or internal mandrel halves is of a correspondingly complex construction and thus entails relatively high investment costs. Moreover, the surfaces of the die or the mandrel halves are constantly damaged due to their movement relative to the blank. The surface defects then lead to further faults in the subsequent processes, for example to the sheet metal blanks becoming jammed in the form gap.
In addition to this, German patent DE 966 111 discloses a device for bending sheet metal strips round to form pipes of a large diameter, which device consists of two vertically opposite coreless cheek plates having a semi-circular cross-section which, in the starting position, encircle with their lower edges the longitudinal sides of the sheet metal strip over the entire length thereof and, on meeting, roll the sheet metal strip into a pipe. During this forming process, the sheet metal strip is firmly clamped along the central longitudinal axis by a clamping device. However, in the known operation of bending sheet metal strips round into pipes without an internal mandrel, the butt joint, required for welding, of the mutually opposite longitudinal edges of the formed metal sheet is not guaranteed for achieving a constant weld seam, so that a reliable straight bead welding of the hollow sections is not guaranteed. Furthermore, the pipes produced in this manner have to be subjected to an additional calibrating procedure.